1. Field of the Invention
The present invention is in the field of radioisotope production and in particular relates to a method of producing strontium-89.
2. Description of the Prior Art
Radioisotopes have been used in nuclear medicine for diagnostics and therapeutics for more than fifty years. Medical radioisotope production is an important industry using more than 50% of the radioisotopes produced in the world. More than 160 radioisotopes of 80 chemical elements are produced with the help of nuclear reactors and charged particle accelerators today.
One of the most effective modern therapeutic radioisotopes is strontium-89. It is used for pain palliation instead of drugs when treating cancer. When medicine containing strontium-89 is introduced into an organism, it is absorbed and distributed in the bone metastases providing for a long anesthetic effect.
Strontium-89 radioisotope has a half-life of 52.7 days with xcex2 decay (decays to Y89, a stable isotope). The maximum energy of the xcex2-particles is 1463 keV. The attendant y-radiation energy is 909.1 keV.
Strontium is a biochemical analog of calcium that has the same transport mechanism in the human body. Strontium chloride SrCl2 introduced to the vein is mainly accumulated in bone metastases providing for a long anesthetic effect so it is not necessary to take drugs frequently and the patient does not become tolerant of them. Malignant tumors tending to metastases in the skeleton are: mammary gland, large intestine, thyroid gland, prostate, kidney, and skin cancer. The maximum range of xcex2-particles of strontium-89 in the bone does not exceed 7 mm, so its radiation effects are isolated to the small area of the skeleton and its radiation burden on the marrow and nearby soft tissue is not significant. As strontium-89 is incorporated in the mineral structure of the bone, diseased metabolism does not take place, and it remains there for more than 100 days. Healthy bone contains a small component of the injected dose and loses it quickly during the first fortnight. One injection of strontium chloride is about 4 mCi and is effective for 3 to 6 months. Clinical tests of the preparation based on 89SrCl3 showed that 65-76% of the patients said that pain had been reduced significantly, and there was full anaesthetic effect in 20% of the cases. In addition, doctors think that strontium-89 chloride has a therapeutic effect, which means it does not only block metastases but also reduces them.
One reactor method of strontium-89 production consists of irradiating a target of strontium carbonate SrCO3 with neutrons having a thermal neutron spectrum. A target made from metallic strontium is irradiated by the neutron flux of a nuclear reactor. Natural strontium consists of the following isotopes: Sr84 at 0.56%, Sr86 at 9.9%, Sr87 at 7.0% and Sr88 at 82.6%. The strontium-89 radioisotope is formed in the target as a result of the neutron capture reaction of one of the strontium isotopes Sr88(n,y) Sr89. A highly enriched target containing Sr88 greater than 99.9% is used because it is necessary to eliminate strontium-85 from the reaction Sr84 (n,y) Sr85, an undesirable admixture. This is a convenient production method and takes place in a normal research reactor. The cross-section of the (n,y)-reaction is only 6xc3x9710xe2x88x9227 cm2, however, which restricts the productivity of this method.
Another strontium-89 production method is based the threshold reaction of neutron capture with the emission of a charged particle Y89 (n,p) Sr89. A target containing natural monoisotope Yttrium-89 is irradiated in the neutron flux of a nuclear reactor with a fast neutron spectrum and is subsequently subjected to radiochemical reprocessing for extraction. Strontium-89 production can achieve about 10-15 mCi per gram of yttrium in optimum conditions. The target is a pellet of yttrium oxide Y2O3 of high purity that is pressed and annealed at 1600xc2x0 C. This method produces almost no radioactive wastes and the end-product does not contain harmful admixtures, e.g., the quantity of attendant strontium-90 is less than 2xc3x9710xe2x88x924 atomic percent.
This method has an extremely low productivity due to the small cross-section of the (n,p)-reaction on Y89, less than 0.3xc3x9710xe2x88x9227 cm2 for neutrons of the fission spectrum. It can only occur in reactors with a fast neutron spectrum, and there are few in existence. In addition, yttrium purified without admixtures of uranium should be used (the uranium content in the Y2O3 pellets must not exceed 10xe2x88x925 by mass). Low productivity and the need for reactors with a fast neutron spectrum are the main problems with this method.
There is clearly a need for a more efficient method for the production of strontium-89, particularly one that uses a relatively low power reactor.
A solution nuclear reactor containing a uranyl sulfate fuel solution produces krypton-89 during operation. Krypton-89 is in the form of a gas that bubbles to the surface of the fuel solution and occupies the enclosed volume above the fuel. An inert gas transports the krypton-89, along with other radioisotope fragments, in a sealed system to a trap area where any accompanying relatively short half-life krypton-90 is allowed to decay to strontium-90. The strontium-90 is removed. Then the krypton-89 is transported to a catching system where it remains until it fully decays to strontium-89. The strontium-89 is removed from the inert gas with the help of sorption in a carbon trap or by chemical interaction in an acid environment. The inert gas is returned to the reactor core.